Such a method and such a radar system is in each case known from DE 696 12 252 T2.
In road motor vehicles, radar systems are used for monitoring the vehicle environment, considering applications such as parking aid, monitoring of dead angle, track-change assistance, door-opening assistance, pre-crash sensing for triggering an airbag, belt tightening, roll bar activation, start/stop operation, stop and follow operation or driving operation with distance monitoring and/or distance control (cruise control support).
There are typically several objects and spatially extended objects (vehicles, crash barriers, bridges) present in the environment of the vehicles so that, as a rule, several reflection points occur in the same distance cell. To enable the driver or a driver assistance system to estimate the situation, the speed and possibly also the angular position of reflecting or scattering objects must also be determined in addition to the distance. A suppression of infrastructure, that is to say, for example, of stationary objects such as crash barriers at the edges of the carriageway, represents a further important requirement for a motor vehicle radar system.
The motor vehicle radar system initially mentioned uses a segmentation of a coverage area of the radar system by means of a sequential overlay with a number of part-areas to which in each case an angular segment from the coverage area is allocated. The segmentation is done with the aid of several spatially separate receiving antennas which can be connected together in pairs in each case. By combining different receiving antennas of the radar system, different part-areas are generated in DE 696 12 252 which, together, produce a comparatively large azimuthal coverage area.
Within a part-area, an FMCW (frequency modulated continuous wave) method is used. In this method, the frequency of the radar signals emitted by the radar system is increased and reduced again in a ramp form. Each individual frequency ramp is also called a chirp. Radar signals reflected from an object and subsequently received by the radar system are mixed down into the baseband with the instantaneous transmit signal in the radar system. For an object in the coverage area of the radar, a harmonic oscillation with a frequency which corresponds to the difference between the instantaneous transmit signal frequency and the received signal frequency (before the mixing) is obtained in the baseband received signal. Since a signal propagation time which depends on the distance r of the radar system from the reflecting object elapses between the sending and the receiving of the radar signals, the distance r is reflected in this difference frequency. Due to the Doppler effect, the radial relative speed vr, that is to say that present in the direction of a connecting line between the radar system and reflecting object, is also reflected in the difference frequency f. In this context, the two dependencies satisfy the linear equationf=a*r+b*vr, with coefficients a and b which depend on the chirp parameters transmitting start frequency f0, chirp length TC and frequency change dfC. The values of the difference frequency f are usually determined by Fourier transform and threshold value detection in the frequency domain.
By means of the linear relationship f=a*r+b*vr, each measured value of the difference frequency f can be represented as a straight line in an r, vr diagram, the slope of the straight line depending on the coefficients a, b and thus on the said chirp parameters. With a ramp-shaped increase in frequency (up-chirp), a different straight line is obtained than with the ramp-shaped reduction in frequency (down-chirp). The intersection of the two straight lines provides the required distance and required relative speed of a single reflecting object. A linear system of equations describes the relation between the object parameters r and vr and the difference frequencies determined. This is already described, for example, by DE 2 305 941.
In situations with a number of reflecting objects, the number of intersections increases with the number of reflecting objects. Each further object provides, in addition to the informative intersection of its two associated straight lines, further intersections with straight lines which belong to the other objects. Since these further intersections do not characterize a real object, they are also called ghost targets.
In such situations with n objects, it becomes necessary to validate the significant intersections, that is to say the r, vr combinations of real objects and thus to separate them from the r, vr combinations of the ghost targets.
For such a validation, it is known from DE 42 44 608 A1 to carry out further measurements which supply additional relationships between the object parameters r and vr and the measurement values for the validation. In this context, a use of further frequency ramps with different slopes is known from DE 199 22 411 A1. In this case, it is checked whether the expected frequencies are measured for a physically possible r, vr combination in the further frequency measurements. This is checked with Nearest Neighbour and Gate methods. An r, vr combination is validated if a difference between an expected frequency and the nearest measured frequency is less than a predetermined threshold value. Following this, the validated r, vr combinations are processed further in the so-called target tracker. In this context, the tracker detects and suppresses individual incorrect r, vr combinations (that is to say ghost targets) by means of an additional validation algorithm.
In DE 696 12 252 T4, it is possible to switch between the individual combinations of receiving antennas by means of which part-areas of a relatively large coverage area are selected. To detect objects in the entire coverage area, the individual part-areas have to be selected sequentially. The duration of an individual measuring cycle by means of which the coverage area is checked once for objects is determined by the product of the measured period required per part-area and the number of part-areas.
In the abovementioned applications for road motor vehicles, a high update rate and thus a short period of a measuring cycle of the order of magnitude between 10 ms to 100 ms is demanded, among other things. On the other hand, the Doppler frequency, and thus the speed of an object, can be determined all the more accurately the longer the frequency ramps of individual chirps are. The more different chirps are used for measuring and validating, the more reliably distances and speeds of real objects can be validated in order to distinguish them from ghost targets.
The requirements for a high update rate and for accuracy of the measurements and reliability of the validation thus result in a target conflict.
Against this background, the object of the invention consists in specifying a method and a radar system of the type in each case mentioned initially by means of which a high measuring accuracy and reliable distinction of real objects from ghost targets can be achieved with a high update rate.